This invention relates to an electrographic apparatus having a transparent photoconductor and a densitometer for measuring the density of toned images on the photoconductor.
Apparatus disclosed in U.S. Pat. No. 4,277,162, issued July 7, 1981, measures the density of toned images produced by electrographic apparatus and transferred to a belt. The apparatus includes a sensor and one or more special light sources which either reflect light rays off of the toned image to the sensor or project light rays through the toned image and the belt to the sensor. The output of the sensor corresponds to the amount of light transmitted through or reflected off of the toned image. The output of the sensor is coupled to a control circuit used to control process parameters in the electrographic apparatus.
Prior electrographic apparatus having densitometers present certain problems. Some apparatus use a small light source, such as a light emitting diode (LED) which is provided solely for the purpose of measuring density or some other parameter. Some apparatus rely on a light scattering principle, thus requiring a relatively large area photodiode or other photodetector to receive light rays from the light source. Apparatus using the scattering principle are sensitive to the position of the photoconductor relative to the light source and photodetector. As a result, the position of the photoconductor must be accurately controlled relative to the light source and detector. The position of the photoconductor frequently is carefully controlled in apparatus where the photoconductor comprises a coated drum or an endless belt. However, the position of a photoconductor in the form of a sheet is more difficult to control.